Functional fluids comprise a broad range of lubricants that are used in automotive and industrial hydraulic systems, automotive transmissions, power steering systems, shock absorber fluids, and the like. These fluids transmit and control power in mechanical systems, and thus must have carefully controlled viscometric characteristics. In addition, these fluids may sometimes be formulated to provide multigrade performance so as to ensure year round operation in variable climates.
Power Steering Fluids (PSF) are one of the most common functional fluids, and an integral part of all power steering systems. Power steering is used in about 80% to 90% of all vehicles in North America and Japan and their use is becoming more commonplace in other parts of the world. These systems are considered xe2x80x9csafety sensitivexe2x80x9d and the major OEMs have stringent specifications to control all aspects of the components that go into their manufacture, including the fractional fluid.
A power steering system comprises a pump, gears, output drives and hydraulic system. The PSF acts as a hydraulic fluid to transfer power in the system and thus assist the driver to control the vehicle. Loss of control could lead to an accident and thus the fluid must have the right viscometrics at ambient start-up temperatures, while maintaining sufficient viscosity at higher operating temperatures. PSF must also be very oxidation stable since it is subjected to high temperatures and is expected to remain in service for up to 100,000 miles in some cases. In addition, and perhaps amongst the most important requirements for a power steering fluid is compatibility with seals and hoses, which can be measured in various ASTM tests, for example, D 471.
In the past power steering fluids generally used blends of naphthenic and solvent neutral base stocks, and their use is still common in many applications. However, over the past few years, with the increasing performance demands being made on power steering fluids, the use of xe2x80x9cnext generationxe2x80x9d hydrocracked base stocks could offer oxidation advantages. However, such molecular compositional changes would not be expected to be compatible with the seals and hoses in power steering systems.